A segmented focusing mirror for use in a passive infrared intrusion detection device is well known in the art. See, for example, U.S. Pat. No. 3,703,718. In a segmented focusing mirror, a mirror has a plurality of segments. Each of the segments receives infrared radiation from a projected finger like region, spaced apart from one another. Each of the segments has a focal length, which is different from others. The plurality of segments all focus the infrared radiation received onto a common focal point where a detector is located.
The degree of spread or divergence of each of the finger like regions is controlled by the focal length of its associated segment. To receive a wide projection from a finger like region, a shorter focal length for a segment is required. Shorter focal length is usually used with segments that are directed to detect intrusion near the detector. In contrast, a finger like region having a narrow width projection, for receiving infrared radiation from far distance, a longer focal length mirror segment is required.
In the prior art, a segmented focusing mirror having a plurality of different focal lengths of the type disclosed in U.S. Pat. No. 3,703,718 has been made by simply constructing a master mirror having the complex requirement of different segments having different focal lengths and yet with all the radiation from each of the segments focused onto a single focal point. A mold is made from this master mirror from which other mirrors are replicated. If errors are made in the master mirror in which either the radiation from the different segments are not focused at the same common focal point and/or the focal lengths selected for one or more of the segments is incorrect in that the finger like regions is projected either too wide or too narrow, then a new master mirror must be re-tooled. This has been expensive.
Fresnel lens has been used also for many years. The drawback of using a fresnel lens to focus radiation is that not all the segments are at the same distance from the detector.